1. Field of the Invention
The present invention relates to a method for producing a methyl methacrylate polymer. More particularly, it relates to a method for producing a methyl methacrylate polymer, including the step of recycling monomers and mercaptan compound as a chain transfer agent, the method comprising recovering a volatile content containing unreacted monomers from a methyl methacrylate polymer composition obtained by a bulk polymerization or solution polymerization method for methyl methacrylate, recovering mercaptan compound from the residue and using the mercaptan compound as a part of polymerization raw materials.
2. Description of the Related Art
As a polymerization method of a methyl methacrylate monomer, a batch suspension polymerization method using an aqueous medium has generally been used. Recently, there has been used a bulk polymerization or solution polymerization method, which is excellent in view of a problem of waste water disposal caused by the aqueous medium, productivity and energy.
As the bulk polymerization method, various methods are disclosed in Japanese Patent Kokoku Publication Nos. 52-32665, 59-21325, 59-21326, 1-49295, 2-26642 and Japanese Patent Kokai Publication No. 3-111408.
The solution polymerization method is disclosed in Japanese Patent Kokoku Publication No. 55-7845 and Japanese Patent Kokai Publication No. 58-132002.
In these polymerization methods, all monomers are not polymerized but the content of the polymer is from about 30 to 80% by weight and a liquid polymer composition comprising an unreacted monomer and a solvent is generally isolated from the polymerization system.
Japanese Patent Kokoku Publication No. 54-42035 discloses to reuse unreacted monomers and a part of impurities having a boiling point of not more than 200.degree. C. Since mercaptan compound used as a chain transfer agent is high-boiling compound compared with the unreacted monomer, the mercaptan compound is generally discarded without recovering as described in Japanese Patent Kokai Publication No. 3-111408.
In such way, since an active component such as mercaptan compound as the chain transfer agent is contained in the residue formed after recovering monomers from the liquid polymer composition, it is discarded.
In view of a reduction in amount of the monomer and chain transfer agent to be used, it becomes necessary that the mercaptan compound is recovered from this residue and reused as the chain transfer agent of the raw material. However, when the mercaptan compound is continuously reused, impurities accumulate to exert a bad influence on polymerization itself and to cause coloring of the resulting polymer.
Therefore, the present inventors have intensively studied about a method for producing a methyl methacrylate polymer, which can efficiently remove impurities contained in the recovered mercaptan compound to perform purification and maintain the quality of the resulting polymer good even if the polymerization is continued for a long time. As a result, the present inventors have found that a methyl is methacrylate polymer having good quality can be obtained even if the polymerization is continued for a long time, by separating a volatile content from a liquid polymer composition to recover unreacted monomers, distilling the volatile content to recover unreacted monomers, (1) subjecting the residue to a contact treatment with hydrazine compound or (2) subjecting the residue to an azeotoropic distillation with water to remove impurities, distilling the resultant to recover the mercaptan compound as the active component and reusing the mercaptan compound as a chain transfer agent of raw materials. Thus, the present invention has been accomplished.